Internalizing disorders (ID) represent the largest domain of emotional disturbances that affect the general population. In recent years, significant effort has been put into examining and defining basic dimensions of functioning (e.g., neural or physiologic) that cut across internalizing disorders as traditionally defined by current nosology. Thus, the goal of the current study is to examine relationships between measures constructed to probe negative valence system (NVS) expression (e.g., fear, anhedonia) in a genetically informative adolescent twin sample. This design allows for the examination of the interplay between genetic and environmental factors as a way of elucidating causal mechanisms involved in the NVS and the role the NVS plays in pathways to internalizing symptoms and syndromes. Within this design we will administer an informative suite of well-validated dimensional measures that tap into the NVS, focusing on five related constructs including anxiety, fear, stress, sadness/anhedonia, and irritability. The influence of genes on psychopathology changes such that different developmental stages are associated with a unique pattern of risk factors representing a dynamic interplay between development, genes, and environment. For this reason, we will target a critical developmental period, focusing on the transition that begins during the late teen years and proceeds into young adulthood. This transition will involve moving away from home/family and established peer networks for approximately half of the general population. For these individuals, a number of significant environmental changes will occur that will impact their emotional functioning and trajectory of internalizing symptom expression. Thus, measuring NVS before this unique developmental period from a genetically informed perspective is ideal for determining the shared and specific contributions of genes and environment to NVS expression and its influence on ID development.